Circulating Thyronines and Peripheral Monodeiodination in Lactating Rats*

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Abstract
The well-known metabolic and endocrine adaptive responses accompanying lactation include a change in circulating thyronines that resembles the so-called euthyroid sick syndrome. To analyze the role played by tissue monodeiodination in this state, circulating levels of thyronines as well as hepatic and mammary 5''-monodeiodinative activity (5''MA) were assessed during lactation in rats. Results show that the serum iodothyronine changes that accompany lactation are associated with a significant decrease in hepatic 5''MA and a simultaneous increase in mammary 5''MA. These changes begin within the first postpartum day, are proportional to lactation intensity (litter size), and disappear 48 h after either precocious (1st postpartum day) or natural (21st postpartum day) weaning. These data demonstrate that the compartmentalized change in energy expenditure characteristic of lactation is accompanied by organ-specific and opposite adjustments in hepatic and mammary monodeiodinative pathways.